Along with recent miniaturization of cameras, proximity sensors, direction sensors, acceleration sensors, angular rate sensors, illuminance sensors, and the like, portable electronic apparatuses, such as a smartphone, are equipped with various sensors as described above. Especially, an electronic apparatus having a liquid crystal panel can measure ambient brightness with an illuminance sensor and appropriately adjust the luminance of a backlight in accordance with the ambient brightness. For implementation of illuminance measurement with a spectral sensitivity characteristic close to visual sensitivity, a technique is known for providing a plurality of photodiodes different in spectral sensitivity characteristic in an illuminance sensor and computing photocurrents in the photodiodes.
Wristwatch-type terminals and eyeglass-type portable electronic apparatuses as terminals secondary to a smartphone are in practical use, and there is already an environment in which biological information, such as a heart rate and the amount of exercise, of a wearer can be monitored and managed at all times. By equipping a portable electronic apparatus of this type to be used outdoors with an ultraviolet sensor and measuring the intensity of ultraviolet rays contained in sunlight, it is possible to promote prevention of sunburn and record the cumulative amount of ultraviolet rays in the daytime. This allows management of health information of a user with use of a portable electronic apparatus.
PTL 1 discloses an ultraviolet measurement apparatus including separate light receivers, separate voltage detection circuits, and separate optical sensor windows for a UV sensor and an illuminance sensor. PTL 2 discloses an optical sensor into which a visible light sensor and an ultraviolet sensor are integrated using an SOI substrate.
In order to minimize optical sensor windows and improve design in a portable electronic apparatus including an illuminance sensor and an ultraviolet sensor, an attempt has been made to cause a single common sensor window to function as a sensor window for the ultraviolet sensor and a sensor window for the illuminance sensor. A photodiode using a GaN-based or ZnO-based compound semiconductor and an SOI substrate has generally been used to detect ultraviolet rays. The use of the compound semiconductor and the SOI substrate leads to the difficulty of integration with a signal processing IC on a single chip and a high cost.
There is a method that measures the intensity and illuminance of ultraviolet rays by arranging a UV (ultraviolet) cutoff filter above one of two light-receiving elements structured such that a plurality of junction photodiodes having different spectral sensitivity characteristics are arrayed in a vertical direction and calculating a difference in signal intensity between the two light-receiving elements.
The difference method allows implementation of provision of an ultraviolet light receiver at low cost. A PN junction photodiode at an outermost surface which has sensitivity to an ultraviolet region is used to receive ultraviolet rays. Since a UV cutoff filter transmits visible light and infrared light, use of a PN junction photodiode at a deep position which has sensitivity to a visible light region or an infrared region allows measurement of illuminance.
Additionally, since the difference method can use a general-purpose silicon substrate, cost can be reduced through integration with a sensor circuit formed by a silicon complementary metal-oxide semiconductor (CMOS) process on a single chip. In addition, ultraviolet rays and illuminance are detected with light-receiving elements as described above which are structured such that a plurality of junction photodiodes are arrayed in the vertical direction, and optical sensor windows for an ultraviolet sensor and an illuminance sensor of a portable electronic apparatus, such as a smartphone, can be integrated into a single common optical sensor window.
Meanwhile, in the case of a light receiver which does not use the difference method and uses a filter transmitting light in the ultraviolet region, it is generally difficult to make sensitivity to a wavelength not less than 400 nm absolutely zero. If an interference film filter is used as a filter transmitting light in the ultraviolet region, the number of layers is larger, and the cost is higher, compared to the case of a UV cutoff filter.
In a case without the difference method, since a filter transmitting only light in the ultraviolet region is used, light in the visible light region is hardly transmitted. For this reason, an optical sensor window for an illuminance sensor needs to be prepared separately from an optical sensor window for an ultraviolet sensor to detect visible right in a wavelength range of 400 nm to 700 nm for the illuminance sensor.
In the field of illuminance sensors, a method that computes photocurrents in a plurality of photodiodes different in spectral characteristic is generally performed to achieve a spectral characteristic close to visual sensitivity. A cross-sectional view of a conventional illuminance sensor using the above-described method is shown in FIG. 24, and a spectral sensitivity characteristic of the conventional illuminance sensor is shown in FIG. 25.
As shown in FIG. 24, the illuminance sensor includes two light-receiving elements (PD1 and PD2) different in spectral sensitivity. The light-receiving element PD1 and the light-receiving element PD2 each have a three-layer structure with a P-type diffusion layer (P+), an N-type well layer (N-Well), and a P-type substrate (P-Sub) and include two photodiodes (PD_vis and PD_ir) composed of a PN junction. In the light-receiving element PD1, the P+ layer and the P-Sub are grounded (GND). In the light-receiving element PD2, the P-Sub is grounded, and the P+ layer and the N-Well layer are connected to each other.
As shown in FIG. 25, a spectral sensitivity characteristic denoted by PD_clear (PD_vis+PD_ir) is achieved by the light-receiving element PD1, and a spectral sensitivity characteristic denoted by PD_ir is achieved by the light-receiving element PD2. Computation of PD1 (PD— clear)−PD2 (PD_ir) allows calculation of a spectral sensitivity characteristic corresponding to PD_vis. The spectral sensitivity characteristic is a characteristic close in peak sensitivity to visual sensitivity, and illuminance can be measured.
It is ideal for an ultraviolet sensor and an illuminance sensor using the above-described difference method that light evenly enters a plurality of light-receiving elements.
Unevenness of radiated light due to an angle for light to be radiated can be reduced to a certain degree by arranging the light-receiving elements such that the light-receiving elements are evenly distributed. However, since the arrangement of the light-receiving elements is fixed, radiated light becomes uneven due to, for example, the angle for light to be radiated and a package surface state when the sensors are sealed with resin to cause variation in spectral sensitivity characteristic. For example, in the case of the illuminance sensor shown in FIG. 24, illuminance is computed through subtraction for output currents from the light-receiving elements PD1 and PD2. If light radiated to the light-receiving elements PD1 and PD2 is uneven, illuminance measurement is unevenly performed to cause variation in spectral sensitivity characteristic.
PTL 3 discloses a method for measurement without unevenness in measurement results and variation in sensitivity by interchanging spectral characteristics for two light-receiving elements arranged at predetermined positions during measurement. For example, a light-receiving element PD1 and a light-receiving element PD2 are respectively set to have the spectral characteristic denoted by PD_ir and the spectral characteristic denoted by PD_clear during a first measurement time period, and the light-receiving element PD1 and the light-receiving element PD2 are respectively set to have the spectral characteristic denoted by PD_clear and the spectral characteristic denoted by PD_ir during a second measurement time period.
PTL 4 discloses a method for correcting measurement unevenness and variation in sensitivity using a corrective photodiode in ultraviolet intensity measurement.